This invention relates to protective relays for electric power generation and distribution systems, and more particularly to an improved current differential protective system of the type disclosed and claimed in U.S. Pat. No. 2,548,625, Seeley.
Heretofore current differential protective arrangements have been commonly used to protect electrical apparatus. For instance in power bus installations wherein a number of source and feeder lines are connected to the bus through circuit breakers it has been the practice to connect current transformers in each source and feeder line and to connect all of the current transformer secondaries in parallel. Appropriate circuitry responsive to the current flow in the transformer secondaries detects the occurrence of an internal fault condition.
In accordance with one embodiment described in the Seeley patent, a circuit consisting of a voltage responsive relay in series with a tuned circuit resonant at the fundamental power frequency, is placed in parallel with the current transformer secondaries. The voltage responsive relay will pick up when an abnormal voltage such as that caused by an internal fault condition appears at the current transformer secondaries. A voltage limiting non-linear resistor, such as a thyrite, across the current transformer secondaries limits the voltage that can be developed across the voltage responsive relay. Seeley describes a second embodiment that has a faster response in which an over current relay connected in series with the non-linear resistor picks up as soon as appreciable current begins flowing therethrough. When either the voltage responsive relay or the current relay picks up all of the circuit breakers are tripped and the secondaries of the current transformers are shorted to make sure that the power rating of the current and voltage relays and the variable resistor are not exceeded.
One disadvantage of the Seeley circuit is that it has a slow response to a fault condition. Since the tuned circuit is essentially a filter, there is a delay of 0.05 to 0.10 seconds before there is adequate current to operate the voltage responsive relay. Even though there is no filter in circuit with the current responsive relay, the current relay will have a relatively slow response to the fault current level just above the minimum current required to operate the current relay. According to the Seeley patent this delay is about 0.016 to 0.05 seconds.
Another disadvantage of the circuits described in Seeley occurs when the fault indication relay initiates the tripping of the circuit breakers but all of the circuit breakers do not clear and the fault current falls to a level below the minimum current level required to operate the current sensitive relay. The reduced current causes the current sensitive relay to drop out even though there is a fault condition. In such a case it would be desirable to have the fault monitoring circuit be responsive to a lower level of fault current after a fault has been detected and action has been taken to operate the circuit breakers.
Another disadvantage of circuits that use a thyrite voltage limiting device as described in Seeley is that the thyrite does not have a sharp voltage versus current characteristic. As a result, during the operate time of the protective circuit, the voltage applied might exceed the insulation breakover level.
For a more detailed explanation of the operation and the application of this prior art arrangement see the aforementioned Seeley patent and also a paper entitled "Instantaneous Bus-Differential Protection Using Bushing Current Transformers" by H. T. Seeley and F. von Roeschlaub, 67 AIEE Transactions 1709-1719 (1948).
It is therefore a principal object of this invention to provide an improved current differential internal fault protective circuit.
It is another object of this invention to provide a current differential internal fault protective circuit having a faster response time.
It is another object of this invention to provide a current differential internal fault protective circuit that responds instantaneously to the magnitude of the fault voltage.
It is yet another object of this invention to provide a current differential internal fault protective circuit that uses a first current level for detecting a fault condition and continues to indicate the fault condition until the fault current falls below a second current level significantly lower than the first current level.